1. Field of the Invention
The present invention relates to a nanoimprint technology, and more particularly, to a flexible imprint mold for patterning ultra-fine nanostructures on a planar or curved substrate and the method for fabricating the same.
2. Description of the Related Art
Nanoimprint lithography is one of the promising technologies for patterning ultra-fine nanostructures with a sub-10 nm resolution by high volume and low cost. In nanoimprint lithography, a mold with nanostructures, generally fabricated on stiff materials such as silicon, silicon dioxide, and quartz wafers, is pressed into polymer film to form relief patterns according to nanostructures of the mold. Nanoimprint lithography is described in detail in U.S. Pat. No. 5,772,905. The usual nanoimprint lithography process of pressing the mold into the thin film of thermoplastic polymer involves applying high pressure and temperature that may limit the application of this method. U.S. Pat. No. 6,334,960 describes a process called “step and flash imprint lithography” that utilizes a mold with a relief structure, which is pressed into polymerizable fluid composition at ambient pressure and room temperature. The polymerizable composition is then subjected to conditions to polymerize the polymerizable fluid composition and form a solidified polymeric material. The mold is then separated from the solidified polymeric material such that a replica of the relief structure in the mold is formed in the solidified polymeric materials.
Micro- and nanostructures on curved substrates should be useful in areas requiring lithography fabrication in three dimensions: examples include lenses and optical fibers, electronic devices shaped to reduce the length of interconnects, and devices that conform to space limitations. However, nanoimprint lithography is typically practiced on planar structures. Both the molds and the substrates are planar. It has been difficult to extend current nanoimprint lithography to a curved substrate. The reason is that the stiff mold cannot provide a conformal contact with the nonplanar surface.
Soft lithography is currently one of useful techniques for patterning features and for fabricating structures on the size scale of 500 nm and larger. In contrast to using rigid molds to obtain high resolution in nanoimprint lithography, the most commonly used stamp material in soft lithography is soft and flexible poly(dimethylsiloxane) (PDMS), which enables it to have intimate physical contact with substrates without external pressure. The flexible character of the stamp can provide a conformal contact to curved substrate. (U.S. Pat. No. 6,180,239). However, the extension of this technique to produce sub-500-nm scale features was limited due to the low elastic modulus of PDMS used in fabricating stamps. Capillary forces, self-adhesion, and mechanical stresses produced during soft lithography process can deform the PDMS stamps or even cause parts of them to collapse, which leads to defective and inaccurate pattern transferring. Higher modulus (ca. 8 MPa) poly(dimethylsiloxane) (hard-PDMS) was developed to achieve resolution of soft lithography to sub-100-nm. (Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042-3049). A composite stamp composed of two layers as a thin hard PDMS layer supported by a thick flexible PDMS layer was proposed to combine the advantages of both a rigid layer to achieve high resolution pattern transfer and an elastic support to enable conformal contact. There were still several intrinsic drawbacks to limit its application: (i) cutting and releasing it from the master caused cracking across the face of the stamp; (ii) the stamp would spontaneously break off the master during heating-cooling cycle because of the large difference of thermal expansion coefficient between the two materials; and (iii) external pressure was required to achieve conformal contact with a substrate, which created long-range, nonuniform distortions over the large areas of contact. (Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M., Langmuir 2002, 18, 5314-5320).